1. Field of the Invention
This invention relates to a process for preparing alkoxysilanes and more particularly, to a process for removing inevitable acidic impurities from organic functional alkoxysilanes to obtain substantially acidic impurity-free alkoxysilanes.
2. Description of the Prior Art
It is usual to prepare alkoxysilanes by a process wherein halogenosilanes such as chlorosilanes, bromosilanes, iodosilanes and the like are esterified with alcohols. The alkoxysilanes obtained by the process inevitably contain halogen-containing components, which are difficult to remove by purification through distillation. The presence of the acidic impurities even in small amounts is disadvantageous in that when the alkoxysilane is directly applied onto metal surfaces as a silane coupling agent or even when it is used in the neighbourhood of metals, the impurities cause the metal to be corroded and degrade storage stability because the alkoxy groups are liable to be hydrolyzed by the action of the impurities. In addition, such impurities will bring about changes in quality with time after molding and curing of the alkoxysilane.
For instance, there is known a technique of preparing so-called water-crosslinking resins which are obtained by subjecting alkoxysilanes to grafting with polyolefins or acrylic resins or copolymerization with olefins or acrylic monomers. If alkoxysilanes containing small amounts of acidic impurities are used for this purpose as they are, the resultant water-crosslinking resin undergoes crosslinkage during storage with the possibility that the molding is not possible or the resin is gelled. This is because the acidic impurities contained in the alkoxysilane serve as a catalyst for hydroysis of the alkoxy groups.
In order to overcome the above disadvantages, there have been proposed several methods of removing the small amount of acidic impurities from the alkoxysilanes. Such methods include (1) a method wherein agents for catching up halogen-containing components such as tertiary amines or urea are used for re-esterification, (2) a method using sodium alkoxides for neutralization, and (3) a method wherein epoxy compounds are reacted to catch up the acidic impurities.
However, the re-esterification method (1) is disadvantageous in that because the produced salts are soluble, the acidic impurities cannot be completely removed even when the re-esterification procedure is repeated several times. With the neutralization method (2), although the small amounts of acidic impurities can be removed efficiently, this method cannot be applied to alkoxysilanes having functional groups, such as an acrylic or methacrylic group and an epoxy group, which are liable to undergo nucleophilic reaction. Further, with the method (3) using the reaction with an epoxy group, halogen atoms are taken in the intended alkoxysilane, with the attendant problems with respect to the degradation with time and the purity after purification.